The brain is composed of intricate circuits of neurons communicating via fast electrical signals created by the coordinated actions of excitatory and inhibitory neurotransmission. Overlaid on this broad structure is a diverse set of slower instructive chemical signaling, referred to collectively as neuromodulation, which critically regulate fast transmission and neuronal function. This proposal brings together molecular-genetic tools, expertise in manipulating and interrogating neuromodulatory neuronal circuits, imaging and electrophysiology to decipher neurohypophyseal regulation of dopaminergic neurons. Dopamine is an essential modulator, required for vertebrate life. Dopamine dysregulation, best studied in degenerative disease, is also associated with anxiety and mood disorders, as well as neurodevelopmental diseases and addiction. Oxytocin, a neurohypophyseal hormone and neuromodulator implicated in social affect and reproductive behaviors, interacts with reward systems indirectly, and also by directly regulating the tonic activity of dopamine neurons, as work from our laboratory has recently demonstrated. The control of dopamine signaling by neurohypophyseal peptides represents a powerful regulation of essential adaptive behaviors, which both emphasizes the central importance of these endogenous peptides in development and establishes them as therapeutic targets for ameliorating disease states. The objective for this proposal is to build on our preliminary data in order to better understand the mechanisms and context of direct neurohypophyseal control over DA neuron function. The major overall premise of this proposal is that neurohypophyseal peptides act centrally in midbrain dopaminergic regions regulating cellular activity, synaptic transmission, as well as plasticity, and that this regulation is sex-independent and important in early development. To address several hypotheses deriving from this premise, we synthesize anatomical, electrophysiological, and behavioral assays, with technical innovations ranging from new light-sheet imaging technologies to promoter-driven viruses for orthogonal control of multiple modulatory systems. Carrying out the proposed experiments would advance our conceptual understanding of the complex neuromodulatory systems regulating affect and reward, and it is relevant to numerous mental health, neurodevelopmental and neurodegenerative disorders characterized by dysfunctional neuromodulation.